Authorities in Costa Rica said Tuesday they are investigating the mysterious deaths of about 500 brown pelicans along the country's Pacific coast over the last five days but do not suspect bird flu was the cause. The first dead birds were spotted by a fisherman on Thursday on San Lucas Island, about 10 miles from the coastal city of Punta Arenas. More turned up in the following days at nearby islands and rivers."This is a situation that is enormously worrisome," Costa Rican Environment Minister Roberto Dobles said. "But it is hard to know what happened, and so it is better not to speculate."Investigators were collecting tissue samples from the dead birds, but tests to determine the cause of death may take several days, said National Animal Health Service spokesman Flor Aguero.Coast Guard marine biologist Carmen Castro said investigators do not think the deaths were caused by bird flu, which is primarily spread by migration.Brown pelicans are not migratory birds, and form stable, permanent colonies. They are not considered an endangered or protected species in Costa Rica.Health Minister Maria Luisa Avila said while agriculture and animal health officials are in charge of the investigation, hospitals have been checked for possible cases of diseases like West Nile virus that could infect both birds and humans.Mosquitoes can spread that disease by biting infected birds and then biting humans. Avila said no such cases have been found so far.